1. Field of the Invention
The invention is generally related to the installation of offshore structures and more particularly to the installation of a deck on a substructure offshore.
2. General Background
In the construction and installation of offshore structures used in hydrocarbon drilling and production operations, it is much easier and less expensive to construct a large offshore structure on land and tow it to the site for subsequent installation than it is to construct the structure at sea. Because of this, every attempt is made to decrease the amount of offshore work that may be needed in an effort to minimize the cost of the structure. Regardless of these efforts, however, a certain amount of offshore work is still required in each case.
In the past, when the deck of a large offshore platform was to be installed, it was often found desirable to build the deck as one large component and install it fully assembled by lifting it from the tow barge and placing it upon the substructure. Unfortunately, as the decks became larger and heavier, there were fewer heavy-lift cranes that could handle such a load. If the deck became too large or heavy to be handled by cranes, it was divided into smaller components that were then each individually lifted into place. This prolonged the installation process since multiple lifts were required and, once in position, the equipment on the separate components had to be inter-connected and tested, thereby necessitating a large amount of offshore work.
An alternate method to dividing the deck into smaller components was to build the deck as a complete unit on shore and then skid this oversized deck onto a relatively narrow barge so that the sides of the deck extended beyond the edges of the barge. The barge would then be transported to the installation site where it would be maneuvered between the upright supports of the substructure (thus the need for a narrow barge and for a wide gap between the upright supports of the substructure). Once in place, the barge would be selectively ballasted, causing it to float lower in the water, and enabling the deck to come to rest upon the upright supports of the substructure. Afterwards, the barge would be moved out from under the deck and de-ballasted.
There are a number of disadvantages to this method. It is limited to a substructure with a large open area in its central region near the water line in order to accept the barge. The barge must also have sufficient beam width to provide stability against roll whenever the deck is supported on the barge. Thus, the substructure and barge, as well as the structural efficiency of the substructure and deck, are all interrelated.
The manner of ballasting the barge prior to transferring the deck onto the substructure also posed problems. The ballasting had to occur rather quickly, almost instantaneously, while the deck was properly located and aligned with respect to the substructure. Any sudden wave or wind force could cause such alignment to go astray or the barge's heave could cause damage to the deck or substructure.
With the advent of floating structures, such as spar type structures and TLP's (tension leg platforms), the ballasting of the vessel supporting the deck can not be carried out quickly. A large deck, for example, one that weighs 15,000 tons, will cause the floating substructure to move downward and, unless the floating substructure is de-ballasted to compensate for this increased weight, it will lose freeboard and could sink. To avoid this, large amounts of water must be pumped out of the floating substructure and this must be done rapidly to avoid repetitive slamming between the deck and the substructure if the seas are rough.
Applicants are aware of U.S. Pat. No. 5,403,124, which discloses a semi-submersible vessel for transporting and installing a deck of an offshore platform onto a substructure. The towing vessel is configured with a cutout or opening therein that surrounds the substructure onto which the deck is to be placed.
A disadvantage of the vessel in U.S. Pat. No. 5,403,124 is that it is limited to a certain maximum size of offshore structure in direct relation to the size of the vessel.